Apoptosis in viral infection of the central nervous system and neuronal cultures.

摘要

Viral infections of the central nervous system (CNS) are important causes of human morbidity and mortality, yet the cellular mechanisms underlying virus-induced cell death are poorly understood. We utilized reovirus infection as a model system for studying viral encephalitis and virus-induced neuronal death. Neurotropic reoviruses infect neurons and cause lethal encephalitis in neonatal mice. We established a neuronal cell culture system for investigating reovirus-induced apoptosis in vitro which mimicked the essential features of reovirus-induced neuronal apoptosis as seen in infected mice. Apoptosis is a form of programmed cell death characterized by distinct biochemical and morphologic changes that are executed by apoptosis-specific proteases (caspases). Serotype 3 reovirus strain Dearing (T3D) induces apoptosis in neuronal cultures by triggering death receptor (DR)-mediated apoptosis that is amplified by activation of mitochondrial death signaling which results in activation of the downstream effector caspase 3. DR activation induces caspase 8-mediated cleavage of the pro-apoptotic Bcl-2 family protein Bid. Cleaved Bid translocates to the mitochondria along with Bax and Bim, two other pro-apoptotic Bcl-2 family proteins. Convergence of cleaved Bid, Bax, and Bim at the mitochondria results in release of the apoptogenic protein Smac/DIABLO from the mitochondria into the cytosol. Cytosolic Smac blocks inhibitor of apoptosis (IAP) family proteins relieving IAP-mediated negative regulation of caspases. T3D-induced neuronal apoptosis in vitro is inhibited by blocking activation of DR, caspase 8 or 3, or JNK/c-Jun signaling. The caspase inhibitors ZVAD-FMK and OPH-QVD, and the neuroprotective tetracycline derivative minocycline, also block T3D-induced apoptosis and injury in the CNS of infected mice. We also investigated the role of apoptosis in human CNS viral infection. We found that infected glia in patients with JC virus-associated progressive multifocal leukoencephalopathy and infected neurons in patients with encephalitis caused by Herpes simplex virus or Cytomegalovirus were undergoing apoptosis, suggesting that apoptosis is an important mechanism of CNS injury in humans as wells as experimental reoviral infections. Viral infections of the CNS remain serious diseases that lack reliable and effective treatments. Understanding how viruses kill cells of the CNS will make possible development of novel anti-viral therapies and anti-apoptosic treatment strategies.